Disc drives of the type known as "Winchester" disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
The movement of the heads across the disc surfaces in disc drives utilizing voice coil actuator systems is typically under the control of closed loop servo systems. In a closed loop servo system, specific data patterns used to define the location of the heads relative to the disc surface are prerecorded on the discs during the disc drive manufacturing process. The servo system reads the previously recorded servo information from the servo portion of the discs, compares the actual position of the actuator over the disc surface to a desired position and generates a position error signal (PES) reflective of the difference between the actual and desired positions. The servo system then generates a position correction signal which is used to select the polarity and amplitude of current applied to the coil of the voice coil actuator to bring the actuator to the desired position. When the actuator is at the desired position, no PES is generated, and no current is applied to the coil. Any subsequent tendency of the actuator to move from the desired position is countered by the detection of a position error, and the generation of the appropriate position correction signal to the coil.
When power to the disc drive is lost, servo control of the current flow in the coil of the voice coil actuator is lost. In the absence of DC current flowing in the coil, the actuator is free to move in response to such things as mechanical shock, air movement within the disc drive or mechanical bias applied to the actuator by the printed circuit cable (pcc) used to carry signals to the coil and to and from the heads mounted on the actuator. Since a power loss also means that the spindle motor will also cease to rotate the discs, the air bearing supporting the heads also begins to deteriorate and contact will be made between the heads and the discs. Because of this, it is common practice in the industry to monitor input power to the disc drive, and, at the detection of power loss, to drive the actuator to a park position and latch it there until power to the disc drive is restored.
It is also well known to use the back electromotive force (BEMF) generated by the inertia of the spinning discs to generate the power to move the actuator to a park position, and the park position is typically selected to be at a location which places the heads closely adjacent the hub of the spindle motor. By parking the heads toward the inner diameter of the discs, the amount of power necessary to overcome the frictional drag of the heads on the discs at power-up is minimized.
An alternative approach to protecting the heads and discs in the event of a power loss to the disc drive is to utilize a ramping system closely adjacent the outer diameter of the discs to remove the heads from engagement with the discs. The actuator is parked with the heads supported by the ramps and latched in this position until power to the disc drive is restored. Upon reestablishment of power to the disc drive, the actuator is unlatched, and the heads are loaded back into engagement with the discs onto an established air bearing. In disc drives utilizing such ramp loading and unloading systems, the heads and discs should never come into direct contact.
The principal requirements of an actuator latch mechanism are that it hold the actuator at the park position in the presence of a defined maximum specified amount of mechanical shock during the time interval when power is not applied, and that the latching mechanism be capable of releasing the actuator once power has been reapplied to the disc drive and the spindle motor brought back up to operational speed. It is also desirable if the latching mechanism can be implemented with a minimal cost, both in mechanical and electronic components.
Many forms of latches to hold the actuator at the park position have been used and are disclosed in the art. These include magnetic latches, solenoid-activated latches, shape-memory metal latches and aerodynamically activated latches. For a representative review of several prior art actuator latches, the reader is directed to U.S. Pat. No. 5,612,842, issued Mar. 18, 1997, U.S. Pat. No. 5,581,424, issued Dec. 3, 1996, U.S. Pat. No. 5,555,146, issued Sep. 10, 1996, U.S. Pat. No. 5,365,389, issued Dec. 15, 1994, U.S. Pat. No. 5,361,182, issued Dec. 1, 1994, U.S. Pat. No. 5,313,354, issued May 17, 1994, U.S. Pat. No. 5,262,912, Dec. 16, 1993 and U.S. Pat. No. 5,231,556, issued Jul. 27, 1993, all assigned to the assignee of the present invention and all incorporated herein by reference.
In latching mechanisms used in association with ramps, it is also desirable that the unlatching of the actuator does not require any sudden large acceleration of the actuator, since no servo control of the actuator exists until after the heads are repositioned in cooperative engagement with the discs. It is well known in the industry that the heads must be loaded off the ramps and onto the air bearing above the discs at a relatively low speed, to ensure that the air bearing is not overcome, allowing the heads to contact the disc surfaces. Any such head/disc contact greatly increases the possibility of damage to the heads, the discs or both.
Clearly a need exists for a head loading and unloading ramp system which incorporates a simple latch mechanism for the actuator in a disc drive, which does not require expensive electronic control circuitry, which holds the actuator with sufficient force to withstand the maximum specified mechanical shock and which is easily released when power to the disc drive is restored.